Conventional automotive power steering systems utilize a pump to provide hydraulic fluid to the power steering gear. These pumps are commonly driven by a belt system attached to the engine which varies the pump speed according to engine RPM. These pumps are preferably designed to provide a consistent flow rate of hydraulic fluid to the power steering system independent of the pump speed. The consistent output flow rate when paired with the variable pump speed can lead to large pressure differentials between the pump inlet and outlet. These large pressure differentials are common at high engine and pump RPM. These pressure differentials may result in cavitation within the pumping chamber. Cavitation can cause undesirable high frequency noise and can lead to premature failure of the pump.
A known technique for minimizing cavitation and ensuring consistent flow output is through the use of a bypass valve and a diffuser. The bypass valve channels variable amounts of fluid from the pump outlet back into the pump inlet allowing the pump to produce a constant fluid output flow at variable pump speeds. The diffuser is used to take fluid from the bypass valve, use this fluid to supercharge low pressure fluid in a reservoir, and draw the fluid from the reservoir into the pump raising the static pressure at the pump inlet. Raising the static pressure at the pump inlet reduces cavitation and its undesirable characteristics.
One known technique for raising the static pressure at the pump inlet using a bypass valve and diffuser comprises using the high pressure outlet fluid from the bypass valve and passing it through a diffuser. In this technique, the bypass valve controls the amount of fluid passing through it to produce a consistent fluid output flow from the pump at varying pump RPM. As fluid passes through the bypass valve, fluid is allowed to flow into a diffuser. As the fluid enters the diffuser, it mixes with the fluid contained in a reservoir located at the mouth of the diffuser. The combined fluid passes through the diffuser which transfers the kinetic energy of the fluid into static pressure.
The diffuser, in this known technique, transfers the kinetic energy of the fluid into static pressure by passing the fluid through the throat of the diffuser which increases the fluid velocity, and therefore the kinetic energy. This is due to the reduction in cross-sectional area along the length of the diffuser throat. After passing through the throat of the diffuser, the fluid passes through a length of the diffuser with an expanding cross-section. The expansion of cross-section causes a decrease in velocity and kinetic energy by transferring them into static pressure. Finally, the fluid, with increased static pressure, flows into a contoured plug formed to direct the fluid in the direction of the inlet port.
This known technique is effective in reducing cavitation in hydraulic pumps. It does, however, increase the complexity and cost of manufacturing the pump, requires the machining of additional parts and enlarges the pump profile. The diffuser and contoured plug are manufactured as separate parts. The diffuser and contoured plug are typically formed of high strength materials such as metal to permit proper sealing with the pump housing. Also, the inlet must typically be located on the top of the pump to allow room for the bypass and diffuser chambers. Therefore, there is a need for a design that retains the reduction in cavitation accomplished by the bypass-diffuser methods, while reducing the complexity, cost, number of parts, machining requirements, weight and profile of the pump.